Variable displacement axial piston pumps typically include a barrel having a plurality of piston assemblies slideably disposed in respective bores within the barrel and a swashplate that is in mating contact with the piston assemblies so that the piston assemblies are forced to reciprocate within the bores of the barrel to receive fluid therein and discharge fluid therefrom. The swashplate is secured to the housing of the pump and is selectively pivotable relative to the barrel so that the volume of fluid being discharged therefrom may be controlled. There have been many attempts to control the pressure transition between the point at which all of the fluid has been discharged from the respective bores and the point at which the respective bores are opened to receive more fluid. Likewise, there have been many attempts to control the pressure transition between the point at which the respective bores are full and the point at which respective bores are opened to discharge fluid. In most of these attempts, special slots or holes are provided to controllably interconnect the high pressure side of the pump to the low pressure side and vice-versa to make the pressure transition as smooth as possible. Even with the special slots or holes, energy is wasted during the respective pressure transitions.
As an example, U.S. Pat. No. 5,593,285 describes a pair of valve plates stacked together in a fixed manner to function as a single valve plate. The valve plate assembly defines a restricted flow path for providing initial fluid pressure communication between an approaching piston bore of the rotatable barrel and an inlet port and a discharge port which extends through the pair of plates to equalize the pressure in the piston chamber with the pressure in the inlet or discharge port. However, an axial piston pump using a conventional slotted valve plate, or multiple valve plates in fixed configuration, suffers from a problem of uncontrolled compression and expansion of hydraulic fluid inside the piston chamber as the barrel rotates through the pressure transitions at top dead center (TDC) and bottom dead center (BDC). These uncontrolled pressure transitions occurring within the piston chamber result in a power efficiency loss for the pump and may also increase the noise and vibration of the pump as well.
Furthermore, FIG. 1 illustrates the porting of a conventional valve plate which utilizes slots at TDC and BDC to facilitate the pressure transition of the fluid within the piston chamber as the piston passes from the high to the low pressure side of the pump. Such slots are known to result in an overall power loss in the pump operation due to the uncontrolled compression or expansion of the fluid within the piston chamber to achieve equilibrium as the piston chamber transitions from the low side to the high side of the pump, and vice versa.
The disclosure addresses these and other shortcomings of the prior art.